1. Field of the Invention
The present invention relates to improved keyless chuck tightening and releasing mechanisms used in reversible electric drills and other reversible power tools which may be selectively equipped with alternative workpieces.
2. Description of the Prior Art
Prior U.S. patent application Ser. No. 358,030 describes a system for automatically advancing and retracting drill chuck jaws until resistance is encountered merely by actuation of the power tool motor. No separate manual actuation or manipulation of a chuck key or other removeable tool is required. A clutch mechanism is employed and is actuated at a mechanical advantage to prevent the sudden impact of engagement of clutch interface surfaces. The clutch mechanism of the prior device also is equipped with a spring washer, the force of which is overcome whenever too great a torque exists between the chuck propulsion barrel and the chuck guide. This prevents internal damage to the power tool, since otherwise the chuck jaws could be advanced to squeeze the drill bit until stress failure in the drill mechanism resulted.
One problem which was not recognized in the prior U.S. application Ser. No. 358,030 was that damage to the chuck mechanism can result if the jaws of the chuck mechanism are fully retracted with the full force of the rotary drive shaft. When the chuck jaws are retracted in this fashion, they can become jammed in the chuck.